In UMTS high speed download packet access (HSDPA) and enhanced dedicated uplink transport channel (E-DCH), collectively known as HSPA, use an HARQ scheme within the MAC layer to enhance the efficiency and reliability of data transmission between user equipment (UE) and the base station known as the Node B. This is, for example, defined in the 3GPP TS25.321 version 6.9.0 specification at sections 11.6 and 11.8. In HSDPA, the receiver side (UE) indicates the channel quality to the transmitter (Node B) using a 5 bit channel quality indicator (CQI) based on short term measurements of the pilot channel signal strength. These measurements can take approximately 2 ms.
Based on the reported CQI, the Node B will choose a transport block size and modulation and coding scheme (MCS) for the next transmission to the UE so that the transport block error probability should not exceed 10%.
Data is sent in the high-speed medium access control protocol data unit (MAC-hs PDU). When the UE receives the MAC-hs PDU, it computes a cyclic redundancy check (CRC) to determine correct reception. If the reception is successful, the UE sends an ACK to the Node B. If not, a NACK is sent. If the Node B receives a NACK, it retransmits the MAC-hs PDU again for Chase combining or the systematic and/or parity symbols for Incremental Redundancy if the number of retransmissions is within the specified maximum number as set by system operator. Incremental redundancy or a Chase Combining Scheme is used for the retransmission, as specified in 3GPP TS 25.212, version 6.9.0.
When the maximum number of HARQ retransmissions fail, as referred to herein as HARQ retransmission failure, retransmission of MAC-hs PDU is considered a failure. The acknowledged mode of operation of the radio link control (RLC), as defined in 3GPP TS25.322, version 6.8.0, sections 9.7 and 11.3, recovers the HARQ transmission error by retransmitting the RLC-PDUs originally multiplexed into the MAC-hs PDU which is indicated as not being received by the receiver side. This functionality is generally referred to as Automatic Repeat-reQuest or ARQ. This is based on the transmitter polling the receiver for the status of PDUs that it has or hasn't received and the receiver send this information back to the transmitter for processing. This two way signaling clearly adds a degree of additional latency in the transmission period of the incorrectly received data between the transmitter and the receiver.
In order to reduce this RLC level retransmission latency, the RLC can be notified of the HARQ retransmission failure and the RLC service data unit (RLC-SDU) or part of the RLC-SDU (the RLC-PDUs) can be retransmitted on the notification rather than waiting for a status report control message from the receiving side. An example of this type of enhancement is indicated in the RAN2 meeting number 55 contribution R2-062906 from NTT DoCoMo for the work item “Long-term evolution of UMTS Terrestrial Radio Access (UTRA) and Universal Terrestrial Radio Access Network (UTRAN)” (LTE). The result of 3GPP study on LTE is shown in the 3GPP TR25.912 version 7.0.0 specification. However, second or subsequent attempts may fail again due to the relatively long lasting bad radio conditions due to shadowing or other factors. In particular the size of the RLC-PDU that is re-transmitted is the same as the previous transmissions. In UMTS, the size of this RLC-PDU is determined during segmentation of the RLC-SDU as determined by the RRC configuration of RLC.